Coupled circuit system



. 25 cations.

Patented Nov. 15, 1938 UNITED STATES PATENT OFFICE COUPLED cnwurr SYSTEM I Alfred W. Barber, Flushing, N. Y. Application January 30, 1936, Serial N0..61,458 17 Claims. (01. 1 79-171) This present invention of mine concerns improvements in radio receiver circuits. It particularly relatestomethods of, and means for, the automatic control of radio receiver fidelity as a 6 function of the amplitude of the received signal by automatic control of selectivity in the radio or intermediateffrequency amplifier circuits.

One objectof my invention is to provide a coupled. circuit in which the coupling may be controlled electrically. Another object is to provide in a radio or intermediate frequency amplifier, circuits coupled by'electrically controlled coupling means.- Still another object is to provide electrically controlled coupling means in connectiorrwith coupled circuits in a radio receiver wherebythe band-pass characteristics of the receiver maybe varied A further object is to provide electricallycontrolled coupling means in the coupled'circuits of a radio receiver wherein the 20 electricalcontrol operates from voltages generated by rectification ofa received signal whereby the receiver responselis controlled by the strength of the receivedsignal. These and other objects will be set forth in detailin the following specifi- Most radio and carrier Wave receivers at the present time employ radio and intermediate frequency amplifiers consisting of cascaded thermionic vacuum tube amplifiers interconnected 0bymeansof systems of coupled circuits. In general these. coupled circuits are capacity tuned to a.;desired-frequency. The selectivity characteristicsof each pair of circuits is determined by the power factor of the individual resonant cir- 3511cuits and the degree of coupling between circuits of a pair.

circuits tuned to the same frequency will exhibit asinglejpeak; of maximum response. With more than critical coupling between circuits of a pair 40- a double peaked or band-pass response is produced when eachfcircuit is tuned to the same frequency. Until recently most receivers have been designed with very nearly critical coupling between interstage circuits in order to obtain maximum selec- 45.: tivityg and gain. Such receivers, however, at-

tenuate the higher modulation frequencies, the cut off often being as low as 3000 cycles. The fidelity may .be. improved by over-coupling the receiver circuits but the better fidelity is usable 60.;only on: strong signals. On weak signals interchannel interference. is experienced with wide band response. receivers. These considerations have led tothe design of receivers having manually controlled circuit coupling for band expanba sion under favorable receiving conditions. In

With less than critical coupling, two

general the manual control is designed to vary the coupling from critical or acondition of maximum selectivity to something greater than crit- I ical giving a fifteen to twenty kilocycle band-pass response.

My presentinvention is concerned with means whereby the coupling between interstage circuits and hence the band-pass characteristics of a carrier wave receiver may be varied automatically in accordance with the strength of the received signal. A thermionic vacuum tube exhibits an input capacity which is a function of its plate to grid capacity and its grid to plate circuit gain. The plate to grid capacity may be the internal tube capacity plus wiring and stray capacities or thesecapacities may be enhanced by an addi' tional discrete capacity connected between plate and grid. With a resistance as a plate load the effective input capacity to the tube is approximately the tube gain plus one times the sum of all plate to grid capacities. Since the gain of the tube may be varied by means of the grid bias, this input capacity may be controlled by means of the grid bias. My invention consists essentially in using this tube input capacityas a coupling impedance between interstage tuned circuits and in controlling the impedance and hence the coupling automatically by means of the voltage generated by rectification of the amplified signal. Since the capacity may be electrically controlled byzgrid bias control, the coupling and hence the selectivity or band-pass characteristics of the coupled circuits and the receiver may be electrically controlled. 7

One method which I employ in coupling two circuits is to place this controlled capacity in series with the two inductances of the coupled circuit and ground. Another method which I have used is to place the controlled capacity in a common path with part or all of the capacity of the tuning condensers of the two circuits. In order to control the coupling and hence the selectivity or band-pass characteristics of such coupled circuits in; accordance with the strength of the received signal, I derive at least a part of the coupling tube bias from voltages derived from the rectification of the signal traversing the system. This control voltage may be the same volt age that is used for automatic volume control in the receiver or.it may be a separately derived voltage. Any or all of the radio or intermediate frequency circuits of a carrier wave receiver may i be thus automatically controlled and the characteristics of the receiver varied over wide limits.

The appended claims set forth, in particular,

, the following amplifier tube I0.

60 l the' capacity of condenser I9.

the novel features to be found in this invention. The following description, however, when taken in connection with the drawing, will serve to set forth the theory and mode of operation of my invention.

In the drawing, Fig. 1 shows a carrier wave amplifier circuit embodying one form of my invention.

Fig. 2 shows an interstage coupling circuit embodying another formof my invention.

Fig. 3 shows a pairv of coupled circuits equivalent to the coupling accomplished in'Fig. 1.

Fig, 4 shows the equivalent circuit of Fig. 2.

Fig. 5 shows a series of curves useful in explain- 5 ing the operation of my invention. 7

Fig. 1 shows the application of myautomatic control, in one form, to two stages of a radio or intermediate frequency amplifier. This amplifier may be a part of a tuned radio frequency, superheterodyne or other type of carrier wave receiver or amplifier. The interstage circuit consistsof a coil Igwhich we may call the primary coil, and a. coil 2, which we may call a secondary coil. Primary coil I receives a. voltage from coil 3 coupled inductively to it and connected to plate dofthe tube 5. Tube .5-derives an input on grid Ii from preceding circuits. Coil I is tuned by condenser I "and coil 2V is tuned by condenser '8. The voltage across coil 2iis applied to grid 9 of The coupling impedancebetween coils I and 2 is placed between their common junction point I I and ground G; This couplingimpedance consists of the input impedance of tube I2 which. is predominant- 1y ca capacity reactance. 'Tube' I2 comprises a cathode I4, 9. control grid I5 and a plate or anode I3. LA cathode current bias resistor II by-passed byrcondenser I8 is provided to give the tube an initial bias although other bias means may be IO-"used- A condenser I9 is shown between grid I5 and plate 161' which may betaken to represent the of the internal and external plate to grid'capacity. Resistor 20in the circuit of plate lfiilsprovided as a gain producing load. The input capacity betweengridliand ground Gis approximately equal to the capacity IStimes the-gridto' plate'gain of the tube plus one. The gain ofthetube is a function of the value of resistor, 20:and the net bias on grid I5. If abias is .50-*sup'pliedto grid I5'thru resistor I3 by means of lead 2I,"22b'eing a by-pass condenser, the gain of the tube I2 is altered in accordance with the bias and-ithe effective. input capacity is altered. Making the externally applied bias more negative decreases the gain of tube I 2 and decreases its effective input capacity. With no-externally applied biasrthe effective input'cap acity may be twice oi. more. the capacity of condenser I9 while at a cut-ofibias the input'capacity will be equal to Since the coupling between coils I 'and'2 is the common impedance ofthe input capacity of tube I2, the smaller this capacitythe greater the coupling between the coils'I and 2. Fig. 3shows an equivalent circuit with variable condenser 23 as the common coupling-impedance. In Fig. 1 condenser I9 may be chosen=as the maximum desired coupling impedanceto give maximum band-pass. In one case I found that a band-pass of twenty kilocycles was produced with condenser equal to 1000 micromicrofarads. At cut-oil" bias on grid I5 the effective coupling impedance will be equal to the capacity; of condenser I9. Resistor 2Il may be chosen so that at z'eroexternal applied bias, the gain 'of tube l2 increases the coupling capacity and hence reduces the coupling impedance to give the desired minimum band-pass. Thus by a change in grid bias on tube I2 the coupling and band-pass may be varied between almost any desired limits.

The same band-pass control system is shown between tubes I0 and 25 as was described in connection with tubes 5 and III.

The expansion control voltage is generated by rectification of the amplifier output. Tube 25 feeds a coil 2"! tuned by condenser 28 thru its plate 26. Detector tube 35 receives an input from *coil29 coupled to coil 21 and tuned by condenser 30. Detector 35 comprises a cathode 36 and anodes 3! and 38. Cathode 36 is connected to one endof coil 29 and ground G. The other end of coil 29 feeds anodes 31 and 38, anode 31 thru the output load consisting-of resistor 3| by-passed by condenser 32 and anode 38 thru the output load consisting of resistor 33 by-passed by condenser 34.- The resistor 33 condenser 34'loadmay be made suitable for" generating audio demodulation products and the drop may be applied to an audio amplifier or output device. Resistor III and by-pass condenser 32 should form a load only to very low frequencies and direct current thereby developing a voltage drop suitable for automatic volume and other control purposes. Since anode 31 will become negative with respect to ground when a signal is received, the polarity is correct for automatic coupling control as well as automatic volume 'control. This negatively increasing voltage which is generated upon receivingv a signal may be applied to grids I5 by means of the coupling and filtering'resistors 39 and 24 and'condensers 40 and'22 and thru the grid resistors I3. The greater the magnitude of the received signal, the greater the negative voltage generated and applied to the'coupling tubes and the smaller: the couplingcapacity and the greater the coupling and band-pass.

It will be noted that the same control bias is applied to the control grids I5 of the coupling control tubes I2 and tothe control grids 9 and 4| of the amplifier tubes I0 and 25, thru the coils 2, thus producing automatic band-Width and volume control from the same control voltage. A wide range of modifications are possible as for instance a different bias may be applied to the amplifiers and one which varies in a different way from the bias applied to the coupling tube grids. control different from the coupling control is to use different cathode-resistor combinations in the amplifier and coupling tube circuits. Delayed control may be applied to the amplifier or coupling tubes or both by means of a bias ap plied to the detector which prevents rectification until the inputsignal reaches a predetermined value. v

Fig. 5 shows several curves of total bandwidth in kilocycles vs. signal strength in microvolts. Curve a shows an automatic expansion characteristic obtained with delayed coupling control bias. No expansion takes place until the received signal reaches micro-volts from which point the expansion increases reaching a maximum at about 170,000 micro-volts. Curve b shows a curve of expansion resulting from no bias delay while curve 0 shows an initial rapid increase in expansion due to a control tube with a different grid control characteristic. For present general receiving conditions a curve similar to a seems most satisfactory with the amount of delay depending on local conditions.-

One method of making the amplification coupling impedanceis a capacity. common to part or all of the tuningcapacity of two tuned cir"- 5 "cults; Aninput or primary coilfl," tuned in part by condenser l, is connected to plate 4 of tube 5.] The output. ors'econdary coil 2 is tuned in 1 lowing tube I02 true completion ofthe tuning 'of coil I is accomplished bycondenser 42 in series with the input capacity of coupling tube I2 while the tuning of coil 2 is completed by condenser 43. inlseries with the, input capacity of coupling I tube I2. "Thus the input capacity of tube I2' is I and! and thus forms a common coupling impedance between the coils. Although different values of condenser I9 and resistor 20 may be used in this circuit, the coupling action is similar to that described in connection with Fig. l. The greater the input signal, the greater the bias on tube I2 and the smaller its input capacity. The smaller the input capacity the greater the degree of coupling between coils I and 2 and the greater the band-pass width of response. Grid 9 of the amplifier tube I0 is indicated as controlled by an automatic volume control voltage thru coil 2. Grid I5 and hence the band-pass width is controlled by a separate voltage marked AEC to denote automatic expansion control thru grid resistor I3. The expansion and volume control voltages may be separately generated and may vary in different ways as functions of the signal strength or they may be the same as voltage generated as shown in Fig. 1.

The circuit shown in Fig. 4 is equivalent to the circuit of Fig. 2. Coil I is tuned by condenser I in parallel with condensers 42 and 44 in series while coil 2 is tuned by condenser 8 in parallel with condensers 43 and 44 inv series. Thus condenser 44 being common to both tuned circuits forms a common coupling impedance. In Fig. 2 the input capacity of tube I2 replaces coupling condenser 44 of Fig. 4.

While I have described only a few systems whereby my invention may be carried into efiect and have pointed out a few possible variations, it will be apparent to one skilled in the art that many modifications are possible without departing from its spirit and scope as set forth in the appended claims.

What I claim is:

1. In a radio receiver, an intermediate frequency vacuum tube amplifier including the combination of two resonant circuits between each pair of amplifier tubes and means for coupling said two resonant circuits comprising an impedance common to said two resonant circuits, said impedance comprising the input impedance of a thermionic vacuum tube,

2. The combination as set forth in claim 1 in which the input impedance of said coupling tube is controlled by the amplitude of the signal traversing said amplifier.

3. In a radio receiventhe combination of interstage selective means and interstage coupling means, said interstage selective means comprising pairs of resonant circuits and said coupling onic vacuum tubes connected mutually in series with circuits comprising each pair.

4. The combination as set forth in claim 3 and including means for controlling said input capacities as a function of the amplitude of signals traversing said receiver.

common. toa part .of thetuning capacity of'coils' means comprising the input capacities of thermi- -5. In a carrier wave empiirying system, the combination of a plurality 'of thermionic vacuum tube repeaters; interstage selective means" comprising pairs of j tuned circuits, mutualcou- 1 pling impedances between said circuits compris j ingeach pair, a rectifier receivingamplif edfwaves traversing the system, said. rectifier generatinga unidirectional voltage proportional to the amf plitude of said waves, in which each of said coupling impedances comprises the input impedancefl of a thermionic vacuum tubej and conductive means between said rectifier and the control" grids of said coupling tubes. L e

6. In a carrier, wave amplifier, the combination of at least twotunedjcircuits and acoupling ime pedance common" to atlleast a part of "said two tuned circuits in which said common imp edancje' comprises the grid to cathode impedance "ofa impedance comprising the control grid to cathode I impedance of a thermionic vacuum tube, in which each tuned circuit comprises a coil and condenser, one end of each condenser being connected to one end of each coil, the other end of each condenser being connected to ground, the other end of each coil being connected together and to said control grid.

8. The combination as set forth in claim 7 and including a condenser connected between plate and grid of said vacuum tube and a resistor in the plate circuit of said tube.

9. The combination as set forth in claim 1 and including means applying a bias to said control grid at least a part of said bias being a function of the amplitude of signals traversing said amplifier.

10. In a carrier wave amplifier, the combination of at least three thermionic vacuum tube repeaters each including at least a cathode, control grid and plate, a tuned circuit connected between the plate of the first of said repeaters and a source of direct current voltage, a second tuned circuit connected between the control grid of the second of said repeaters and ground, and a condenser connected between the plate of said first repeater and the control grid of the third of said repeaters and a second condenser connected between the grid of said second repeater and the control grid of the third of said repeaters.

11. The combination as set forth in claim 10 and including a rectifier receiving the output of said amplifier and conductive means betweensaid rectifier output and the grid of the third of said repeaters.

12. The combination asset forth in claim 10 and including a rectifier receiving the output of said amplifier, conductive means between said rectifier and the grid of the third of said repeaters and additional means comprising a resistor in the circuit of the plate of the third of said repeaters.

13. In a carrier wave receiver, the combination of at leasttwo thermionic vacuum tube repeaters, a. tuned circuit associated-with the output circuit of one of said repeaters and a second tuned circuit associated with the input of another of said reazpeaters, a coupling impedance common to at least a partof the impedance of said output and input. tuned circuits, a rectifier receiving) the output of the last of said repeaters and including a direct current output load circuit, in which said lo'rfcoupling impedance comprises the grid to cathode impedance of a thermionic vacuum tube and conductive means interconnecting said grid and said rectifier output circuit.

15. The combination as set forth in claim 14 15g wherein the net bias on said grid is the sum of the drop across said rectifier output circuit and the cathode current drop in a resistor connected betweensaid cathode and ground.

16. In a selective, carrier wave amplifier, means for varying the selectivity of said amplifier comprising a pair of capacity tuned circuits, means for passing a fractional part of the capacity current in said tuned circuits thru a common impedance comprising the inputcapacity of a thermionic vacuum tube and means for varying said input capacity as a function of the amplitude of signals traversing said amplifier.

17. A system as set forth in claim 16 wherein said means for passing a fractional part of the capacity current in said tuned circuits comprise electrical condensers having capacities small compared to the total tuning capacity of said tuned circuits.

ALFRED W. BARBER. 

